In the prior art, it is known that optical information may be recorded or written in a thermoplastic film by controlling the three parameters of the surface charge density, the light intensity and the temperature of the thermoplastic film - see the A. E. Jvirblis, et al, U.S. Pat. No. 3,795,514. More recently, work has indicated that holographic recording on thermoplastic films may be the basis of future high-density holographic storage of analog and digital data. See the publications "Holographic Recording on Thermoplastic Films," T. C. Lee, Applied Optics, Vol. 13, No. 4, April, 1974, pp. 888-895, and "An Experimental Read-Write Holographic Memory," W. C. Stewart, et al, RCA Review, Vol. 34, March, 1973, pp. 3-44.
In the prior art of optical recording in thermoplastic films, a major problem that has been encountered has been the inconsistency of the recording process, i.e., for a given optical signal that is to be recorded, the resulting signal intensity that is obtained from the recorded optical signal varies significantly with each individual recording operation and with time. That is, the thermoplastic film appears to exhibit a material fatigue effect after many, e.g., 1,000, record-erase cycles such that signal intensity not only varies between individual recording operations, because of variations of recording parameters, but also varies after many recording operations because of variation of recording material characteristics. The present invention provides a method of and an apparatus for ensuring that the recording operation may be controlled to provide consistent signal intensity between successive recording operations and after many recording operations.